Chap-23

TCP/IP Protocol Suite*Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Chapter 23Electronic Mail: SMTP, POP IMAP, and MIME

TCP/IP Protocol Suite

TCP/IP Protocol Suite*OBJECTIVES:To explain the architecture of electronic mail using four scenarios.To explain the user agent (UA), services provided by it, and two types of user agents.To explain the mechanism of sending and receiving e-mails.To introduce the role of a message transfer agent and Simple MailTransfer Protocol (SMTP) as the formal protocol that handles MTA.To explain e-mail transfer phases.To discuss two message access agents (MAAs): POP and IMAP.


TCP/IP Protocol Suite*OBJECTIVES (continued):To discuss MIME as a set of software functions that transforms non-ASCII data to ASCII data and vice versa.To discuss the idea of Web-based e-mail.To explain the security of the e-mail system.


TCP/IP Protocol Suite*Chapter Outline23.1 Architecture23.2 User Agent

23.3 Message Transfer Agent23.4 Message Access Agent23.5 MIME23.6 Web-Based Mail23.7 Electronic Mail Security


TCP/IP Protocol Suite*23-1 ARCHITECTURETo explain the architecture of e-mail, we give four scenarios. We begin with the simplest situation and add complexity as we proceed. The fourth scenario is the most common in the exchange of e-mail.


TCP/IP Protocol Suite*Topics Discussed in the Section First Scenario Second Scenario Third Scenario Fourth Scenario


TCP/IP Protocol Suite*Figure 23.1 First scenario


TCP/IP Protocol Suite*When the sender and the receiver of an e-mail are on the same mail server,we need only two user agents.


TCP/IP Protocol Suite*Figure 23.2 Second scenario


TCP/IP Protocol Suite*When the sender and the receiver of an e-mail are on different mail servers,we need two UAs and a pair of MTAs (client and server).


TCP/IP Protocol Suite*Figure 23.3 Third scenario


TCP/IP Protocol Suite*When the sender is connected to the mail server via a LAN or a WAN, we need two UAs and two pairs of MTAs (client and server).


TCP/IP Protocol Suite*Figure 23.4 Fourth scenario


TCP/IP Protocol Suite*When both sender and receiver are connected to the mail server via a LAN or a WAN, we need two UAs, two pairs of MTAs (client and server), and a pair of MAAs (client and server). This is the most common situation today.


TCP/IP Protocol Suite*Figure 23.5 Push versus pull


TCP/IP Protocol Suite*23-2 USER AGENTThe first component of an electronic mail system is the user agent (UA). It provides service to the user to make the process of sending and receiving a message easier.


TCP/IP Protocol Suite*Topics Discussed in the Section Services Provided by a User Agent User Agent Types Sending Mail Receiving Mail Addresses Mailing List or Group List


TCP/IP Protocol Suite*Some examples of command-driven user agents are mail, pine, and elm.


TCP/IP Protocol Suite*Some examples of GUI-based user agents are Eudora, Outlook,And Netscape.


TCP/IP Protocol Suite*Figure 23.6 Format of an email


TCP/IP Protocol Suite*Figure 23.7 E-mail address


TCP/IP Protocol Suite*23-3 MESSAGE TRANSFER AGENTThe actual mail transfer is done through message transfer agents (MTAs). To send mail, a system must have the client MTA, and to receive mail, a system must have a server MTA. The formal protocol that defines the MTA client and server in the Internet is called Simple Mail Transfer Protocol (SMTP). As we said before, two pairs of MTA client-server programs are used in the most common situation (fourth scenario). Figure 23.8 shows the range of the SMTP protocol in this scenario.


TCP/IP Protocol Suite*Topics Discussed in the Section Commands and Responses Mail Transfer Phases


TCP/IP Protocol Suite*Figure 23.8 SMTP range


TCP/IP Protocol Suite*Figure 23.9 Commands and responses


TCP/IP Protocol Suite*


TCP/IP Protocol Suite*Figure 23.10 Connection establishment


TCP/IP Protocol Suite*Figure 23.11 Message transfer


TCP/IP Protocol Suite*Figure 23.12 Connection termination


TCP/IP Protocol Suite*Let us see how we can directly use SMTP to send an e-mail and simulate the commands and responses we described in this section. We use TELNET to log into port 25 (the well-known port for SMTP). We then use the commands directly to send an e-mail. In this example, [email protected] is sending an e-mail to himself. The first few lines show TELNET trying to connect to the adelphia mail server.After connection, we can type the SMTP commands and then receive the responses as shown below. We have shown the commands in black and the responses in color. Note that we have added for clarification some comment lines, designated by the = sign. These lines are not part of the e-mail procedure.


TCP/IP Protocol Suite*23-4 MESSAGE ACCESS AGENTThe first and the second stages of mail delivery use SMTP. However, SMTP is not involved in the third stage because SMTP is a push protocol; it pushes the message from the client to the server. In other words, the direction of the bulk data (messages) is from the client to the server. On the other hand, the third stage needs a pull protocol; the client must pull messages from the server. The direction of the bulk data are from the server to the client. The third stage uses a message access agent.


TCP/IP Protocol Suite*Topics Discussed in the Section POP3 IMAP4


TCP/IP Protocol Suite*Figure 23.13 Pop3 and IMAP4


TCP/IP Protocol Suite*Figure 23.14 Pop3


TCP/IP Protocol Suite*23-5 MIMEElectronic mail has a simple structure. Its simplicity, however, comes with a price. It can send messages only in NVT 7-bit ASCII format. In other words, it has some limitations. Multipurpose Internet Mail Extensions (MIME) is a supplementary protocol that allows non-ASCII data to be sent through e-mail. MIME transforms non-ASCII data at the sender site to NVT ASCII data and delivers it to the client MTA to be sent through the Internet. The message at the receiving site is transformed back to the original data.


TCP/IP Protocol Suite*Topics Discussed in the Section MIME Headers


TCP/IP Protocol Suite*Figure 23.15 MIME


TCP/IP Protocol Suite*Figure 23.16 MIME header


TCP/IP Protocol Suite*Figure 23.17 Base64


TCP/IP Protocol Suite*Figure 23.18 Quoted printable


TCP/IP Protocol Suite*23-6 WEB-BASED MAILE-mail is such a common application that some websites today provide this service to anyone who accesses the site. Three common sites are Hotmail, Yahoo, and Google. The idea is very simple. Let us go through two cases:


TCP/IP Protocol Suite*Topics Discussed in the Section Case I Case II


TCP/IP Protocol Suite*Figure 23.19 Web-based e-mail, case 1


TCP/IP Protocol Suite*Figure 23.20 Web-based e-mail, case 2


TCP/IP Protocol Suite*23-6 E-MAIL SECURITYThe protocol discussed in this chapter does not provide any security provisions per se. However, e-mail exchanges can be secured using two application-layer securities designed in particular for e-mail systems. Two of these protocols, Pretty Good Privacy (PGP) and Secure MIME (SMIME) are discussed in Chapter 30 after we have discussed the basic network security.


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